Variable capacitors (varactors) are widely used in integrated circuits (chips) for clocking and input/output (I/O) applications, e.g., inductor-varactor digitally-controlled oscillators (LC DCOs) to provide a stable desired frequency, or impedance-matching circuits to reduce signal loss at chip inputs or outputs. Tuning is achieved by varying an input control signal to the varactor to change the capacitance value from a low value to a high value and back again. The DCO tuning range is typically divided into overlapping frequency bands (e.g., 16 bands), using a bank of coarse tune varactors to set the band, and each band is subdivided into N overlapping frequency steps, using a bank of fine tune varactors to achieve the exact desired frequency. The fine tune varactor bank is controlled by a digital phase locked loop (DPLL).
The DPLL effectively sends to the varactors a digital word that represents the desired DCO frequency, e.g., the DPLL turns on more of the N fine tune varactors to force the DCO frequency to increase. The representation of this digital word determines the DCO varactor implementation, involving tradeoffs in DCO tuning range and noise performance and varactor on-chip device matching requirements.
In terms of tuning range, using more bits to represent the digital word requires more varactors. Each added varactor adds wiring parasitic capacitance to the DCO LC tank and undesirably reduces DCO tuning range. In terms of noise performance, switching more and larger varactors degrades DCO phase noise performance. In terms of matching requirements, using fewer bits requires tighter varactor device matching, which can be difficult to achieve in chip technologies.
Two examples of digital word encoding schemes typically used in DCO design are binary and thermometer coding (N herein again representing the number of steps in the DCO fine tuning). In binary coding, the varactors are sized in powers of 2, e.g., 1x, 2x, 4x, 8x, where x is a unit capacitance. The number of binary bits required to encode the digital word is minimal (4 in this example), providing wide DCO tuning range, but the varactor matching required is very tight, e.g., varactor 1x and varactor 8x must match (1x must provide ⅛ of the capacitance of 8x). In thermometer coding, each varactor is of identical 1x size. The number of bits required is very large (N), thus limiting DCO tuning range, but the matching required is minimal (1x must match 1x). These two coding schemes thus represent the extremes of the spectrum of coding choices, and neither one is very attractive in terms of DCO performance.